The present invention relates to a contactless IC card for RFID (Radio Frequency Identification) or the like.
Data carrier systems have become widespread, in which an interrogating device (reader/writer) contactlessly induces a voltage in the antenna coil of a responding device (contactless IC card) to send and receive signals.
As shown in FIG. 16, the basic circuit elements of such a contactless IC card are an IC chip and a resonance circuit that is composed of an antenna coil and a capacitor, so that a specific configuration is to arrange the antenna coil, the capacitor and the IC chip on a substrate.
Formerly, rings of thin metal wires that are wound in one plane were used for the antenna coil, but recently, a conductive layer of copper foil or the like that is laminated on one side of an insulating substrate is etched into rings, which improves the antenna characteristics and the mechanical strength and reduces the number of parts to be assembled. Moreover, the insulating substrate on which the antenna coil is etched is used as the dielectric for the capacitor. That is to say, the antenna coil and the capacitor are formed by etching the conductive layer of a laminated board, in which conductive layers are formed on both sides of an insulating substrate.
In this case, a polyimide film is used for the insulating substrate. Furthermore, copper foil adhered with an adhesive, or a copper layer formed, for example, by sputtering is used for the conductive layer.
However, polyimide has a high water absorption rate and its dielectric constant changes as it absorbs moisture, so that the capacitance of a capacitor with a polyimide film as a dielectric changes as it absorbs moisture, and there is the problem that the resonance frequency of the IC card shifts.
In particular, laminated boards in which a polyimide film and a copper foil are adhered to one another with an adhesive have the advantage that they can be obtained at lower cost than those in which a copper layer is sputtered onto the polyimide film, but the resonance frequency of the IC card becomes even more unstable, because the adhesive becomes another cause of changes of the dielectric constant, in addition to the polyimide film.
It is an object of the present invention to solve these problems of the related art, and to provide an IC card with stable resonance frequency and improved antenna characteristics, which can be manufactured at low cost.
The inventors have found that using a mica capacitor having a mica film as a dielectric stabilizes the resonance frequency of the IC card, because the water absorption rate of the mica film is very low, and that putting the terminals of the electrodes on both sides of the mica film together on one side by means of a through-hole, the mica capacitor can be mounted at low cost and with high reliability on a substrate using an anisotropic conductive adhesive; and based on these findings, the present inventors have completed the present invention.
Thus, the present invention provides an IC card having a mica capacitor, an antenna coil and an IC chip formed on a substrate, the mica capacitor being made of a mica film and electrodes formed on both sides of the mica film, wherein a terminal of the electrode on one side of the mica capacitor is formed on the other side of the mica capacitor, and this terminal and a terminal of the electrode on the other side are connected on the substrate by an anisotropic conductive adhesive.